This invention relates to a polyimide-based coating composition for laminating metal substrate such as copper foil. More particularly, this invention relates to a polyimide-based coating composition for making flexible copper laminate that provides improved adhesion, dimensional stability, and surface flatness, and method of applying the same.
Polyimides have been widely used in the electronics industry as a film coating material on metal substrates. Polyimides are well known for their excellent heat-resisting characteristics. However, there exist several areas that need to be improved regarding the application of polyimides in the electronics industry. Typical of these problem areas include: the degree of adhesion between polyimides and the metal substrate (for example a copper foil), the dimensional stability of the electrolytic copper laminate, and the flatness of the copper laminate surface after being clad with polyimides. Surface flatness determines the closeness at which the circuit line can be packed together. To provide a fine line circuit, a highly flat surface will be required.
A common approach to improve the degree of adhesion between polyimides and the copper foil is to use adhesives. Proper selection of adhesives can also help maintaining the flatness of the copper foil surface. However, most adhesives do not have the same extent of heat resistance as polyimides. The use of adhesives often reduces the temperature range of the final product and, therefore, limits the types of applications that polyimides may be used. The additional step of having to apply adhesives before applying the polyimide coating layer also complicates the manufacturing procedure in making copper laminates. Therefore, it is desirable to develop polyimide compositions that provide good adhesion characteristics between the coating layer and the copper foil without the need to use adhesives, and, at the same time, ensure that the final product has acceptable surface flatness.